In a hydraulic CVT, the groove width of a pulley is correlatively changed depending on the hydraulic pressure in a hydraulic chamber, whereby the diameter of the pulley is changed to continuously change a gear. In general, a fixed pulley is integrally formed with a shaft for driving, and a movable pulley is formed in a housing that reciprocates along the shaft. A hydraulic chamber is provided in the movable pulley. The hydraulic pressure in the hydraulic chamber is controlled to allow the movable pulley to be separated from, or be brought into contact with, the fixed pulley. Thus, the width of grooves formed in both the pulleys is increased or decreased to increase or decrease the radius of rotation of a belt wound around the pulleys. This configuration transmits power to change a gear ratio. In order to fill the hydraulic chamber with an oil and generate a hydraulic pressure, a seal ring is attached to a shaft groove formed in the outer peripheral surface of the shaft.
In the CVT described above, when an engine is stopped, an oil pump is stopped. For this reason, a hydraulic pressure is not generated and a load is not applied. In the conventional seal ring, sufficient sealing performance is achieved in a state where a hydraulic pressure is generated, but an adhesion to the inner peripheral surface of the housing is not achieved in a no-load state. As a result, the oil of the hydraulic chamber is escaped. When the engine is re-started in such a state, it takes a long time until the hydraulic chamber is filled with the oil. When the engine is started in a state where the hydraulic chamber is not filled with the oil, a rotating part of the CVT may be damaged by seizure. Therefore, a seal ring which can reduce oil leakage from the hydraulic chamber even in a no-load state where a hydraulic pressure is not generated is required.
As a seal ring for a CVT, a combined seal ring including an endless resin ring 7 that has a substantially rectangular cross section and is disposed on the outer peripheral side and an O ring 6 that is disposed on the inner peripheral side and imparts an expansive power to the resin ring, as shown in FIG. 1, has been used. In general, as a material for the resin ring 7, a polytetrafluoroethylene (PTFE) resin to which a filler is added, or the like, is used, and as a material for the O ring 6, a rubber-like elastic body is used.
In the conventional combined seal ring, the O ring 6 and the resin ring 7 are pushed and collapsed, so as to be installed in a space between a groove bottom 8 and the inner face 4a of a housing 4. Therefore, when a shaft 3 to which the O ring 6 and the resin ring 7 are attached is inserted in the housing 4, the assembly resistance becomes large. Accordingly, it is necessary to introduce a press-fitting device and assemble the housing 4. This causes a problem in which the manufacturing cost is increased and a defect of assembly of a seal ring cannot be detected. In order to solve the problems such as the fitting and cost of the combined seal ring, the application of a one-piece seal ring is required.
Since a maximum hydraulic pressure of about 7 MPa is applied to the hydraulic chamber in the CVT, a seal ring having excellent wear resistance and sealing performance under a high hydraulic pressure is required. In consideration of increase in temperature due to generation of heat during high-speed running and use in the cold district, a seal ring is required to have resistance in a temperature region of −40° C. to 150° C. As a material for the seal ring, a material in which a fluorine-containing resin such as polytetrafluoroethylene (PTFE), modified polytetrafluoroethylene, and ethylene tetrafluoroethylene (ETFE) is charged with an additive such as carbon powder and carbon fibers is used.
For example, Patent Literature 1 discloses a composition, in which carbon black having a certain amount of DBP absorption is blended in a PTFE-based resin, as a resin composition applicable to a CVT. When a seal ring having the composition disclosed in Patent Literature 1 absorbs an oil, the seal ring is swollen and fills a space formed in a radial direction of the seal ring by creep deformation at a high temperature, and the low-temperature sealing performance can thereby be improved. Patent Literature 1 describes that the seal ring has excellent sealing performance even at a low temperature immediately after start of running of a hydraulic pressure device. Further, the seal ring of Patent Literature 1 is used for high contact pressure types such as CVT. Therefore, Patent Literature 1 suggests that, in order to improve wear resistance and creep resistance, carbon fibers and graphite can be blended.
Use of the seal ring of Patent Literature 1 may enable a decrease in oil leakage at a low temperature. However, the above described configuration includes a PTFE-based resin as a main component. Therefore, when a pressure is applied in an automatic transmission fluid at a high temperature, the seal ring is plastically deformed. When an engine is stopped and went into a no-load state after running, it is difficult to maintain the adhesion state (adhesion) to the inner peripheral surface of the housing. Further, it is difficult to prevent the oil leakage from the hydraulic chamber.    Patent Literature 1: Japanese Patent Application Laid-Open No. 2006-283898